The present invention is related to a roll used in paper or paperboard making, on the cylinder shell of which roll is formed a porous coating, and a method for making the roll.
The roll according to the invention can be used for example as a center roll of a press section or as a calender roll, with which a paper web is in direct contact and from which the web is loosened, or for example as a grooved roll or a suction roll (or a wire leading roll, felt leading roll, spreader roll), which is in direct contact with fabrics used on paper machines, such as felts and wires, or as a drying cylinder, a roll of a reeler or a carrying roll.
Coated rolls are used on paper making machines and paper finishing machines in many diverse applications. Such applications include for example press rolls, suction rolls, soft rolls of calenders or supercalenders and the like. Different quality requirements are set for the coating of a roll in different applications and different processes. Different quality factors of a coating include e.g. hardness at a certain temperature, temperature stability, pressing stability, chemical resistance, surface smoothness, resistance to mechanical damage, elasticity, surface energy, loosening properties of paper, electroconductivity and non-aging.
A stone roll made of granite is, as known, used in the press section of a paper machine. Granite is preferred because of its surface properties, which make the paper web loosen from the stone surface in a controlled manner. Furthermore, granite has a good resistance to the wearing effect of a doctor blade. Granite has, however, certain disadvantages. Since it is a natural material, its properties vary, and the internal defects of granite as well as its tendency to crack form a serious obstacle to its use in certain applications. Granite is also a heavy material, which increases the tendency of the structures to vibrate. The weight of a stone roll is also reflected in the dimensioning of the lifting devices and foundations of the paper machine.
Synthetic stone rolls are also known, which in principle correspond to polymer coating rolls, in which stone powder, such as quartz sand, has been added among hard rubber and polyurethane. Excessive adherence of the paper web to the roll surface and poor mechanical resistance represent disadvantages of these rolls.
Finnish Pat. No. 70 723, held by the applicant, describes a press roll, whose surface layer is formed of a mixture of a metallic powder and an inorganic substance. The purpose of the metal is to act as a binder and increase the toughness of the roll coating. The purpose of the inorganic substance is to achieve a wear-resistant surface with a suitable surface energy, since the surface energy of the roll surface has to remain within certain limits to make the loosening of the paper web from the press roll surface in a controlled manner possible. Pat. application No. 853544, filed by the applicant, is also related to a similar roll, in which the metallic component is a chromium-containing stainless steel, the chromium improving the corrosion resistance of the steel.
Finnish Pat. application No. 882006 describes a roll, in which the outer surface of the roll is formed of areas rich in carbide and matrix areas located therebetween.
U.S. Pat. No. 4,704,776 describes a paper machine press roll with a metal body and a metallic basic coating formed on the metal body, whose expansion coefficient is smaller than that of the surface of the metal body, and on which metallic coating is formed a ceramic layer with a porosity of 1-30%.
In prior art rolls, it has always been difficult to achieve sufficiently good, different surface properties simultaneously, e.g. porosity has been a problem. If porosity has been too low, water retention and wettability of the roll surface have remained insufficient, because of which a wet paper web does not loosen uniformly therefrom. If porosity has in contrast been too high, the water retention is too high, which results in a degraded dewatering capacity and as a consequence thereof in too high a water content of paper, poorer strength properties and durability, and finally in an unserviceable roll.
When the purpose has been to produce a surface with a lower porosity for improving the strength properties the pores have been blocked with a plastic sealant by using a brush or by spraying.
In the aforementioned U.S. Pat. No. 4,704,776, this problem has been solved by varying the porosity of the different layers of the coating for adjusting the water retention of the roll.